Prep the pan: Line an 8-inch square pan with parchment, leaving overhang on two sides for easy lifting. Lightly grease the parchment.

Sift the sugar: In a large bowl, sift the powdered sugar to remove lumps. This keeps the fudge extra smooth.

Melt butter and peanut butter: In a medium saucepan over medium heat, add the butter and peanut butter. Stir until fully melted and silky, about 3–5 minutes. Do not boil.

Add vanilla and salt: Remove from heat. Stir in vanilla extract and salt until combined.

Combine with sugar: Pour the warm peanut butter mixture over the powdered sugar. Mix with a hand mixer on low or with a sturdy spatula until no streaks remain. The mixture should be thick, glossy, and uniform.

Transfer and smooth: Scrape the fudge into the prepared pan. Press into an even layer with an offset spatula or the back of a spoon. Tap the pan gently on the counter to release air bubbles.

Optional toppings: Sprinkle flaky salt, crushed peanuts, or mini chocolate chips on top. Press lightly so they adhere.

Set and slice: Chill for 1–2 hours, or let sit at room temperature 3–4 hours, until firm. Lift out using parchment and slice into 1-inch squares with a warm, sharp knife.